The primary goal of this proposal is to characterize the skeletal and metabolic effects of polymorphisms in two adjacent genes in the mid-region of mouse chromosome (Chr) 6, using mouse models created in our laboratory. We initially found a quantitative trait locus (QTL) that strongly influenced peak bone acquisition and IGF-I in a cross between two inbred strains (C3H/HeJ[C3H], and C57BL/6[B6]). We subsequently created a congenic mouse B6.C3H-6T (i.e. 6T) in which this locus was placed on a B6 background. 6T female mice had a remarkable skeletal and metabolic phenotype including very low peak bone mass, reduced bone formation, low serum IGF-I, suppressed leptin concentrations, significant marrow and hepatic adiposity, and insulin sensitivity in response to high fat feeding. In addition, we were able to rescue the trabecular phenotype of 6T by increasing dietary fat intake. We found that the Chr 6 QTL carried at least 6 genes within a very small genomic region that were involved in both adipocyte and osteoblast differentiation, and we showed two of these, Alox5 and Pparg, not only were suppressed in 6T marrow stromal cells (MSCs) but also carried polymorphisms which affected their transcription. Furthermore, we discovered a chromosomal inversion with a break point just distal to Alox5, in the vicinity of a highly conserved region 5'to the stromal differentiation factor-1 (SDF-1) gene. Hence we postulate that the 6T congenic has a functioning regulon, i.e. a genetic unit composed of a non-contiguous group of genes under the control of another regulatory gene, and which is involved in pleiotropic functions. To test that hypothesis, we propose two specific aims: 1-comprehensive metabolic and skeletal phenotyping of 6T, Alox5-/- and B6 mice on high fat diets and after treatment with Pparg agonists;2-delineation of how this regulon influences the Wnt/B-catenin and IGF-I networks utilizing in vitro studies of MSCs, adipocytes and osteoclasts from 6T, Alox5-/- and B6 mice on high fat diets and Pparg agonists. In this 2nd aim we will also determine the in vivo activity of the Wnt/B-catenin system using TOPGAL 6T and B6 reporter mice. Relevance to Public Health Successful completion of this proposal will shed new light on a unique metabolic and skeletal network, and will also provide important insight into the relationship of dietary fat and marrow adiposity to bone acquisition. More importantly, this work could lead to a sea change in our general approach to identifying osteoporosis genes in mice and humans.